Fluid flowmeters of various types are known to the prior art for measuring the flow rate of a moving fluid contained within a conduit. Such flowmeters find particular applicability, for instance, in the aircraft industry in measuring the flow rate of fuel delivered to a jet engine.
The fluid flowmeters with which the present invention is particularly concerned are those including a rotatable measurement assembly contained within a housing which is adapted for interconnection into the conduit through which the fluid is flowing. These flowmeters include volumetric types, in which the measurement assembly comprises a turbine having a plurality of turbine drive surfaces which are angled with respect to the direction of fluid flow through the housing so that the turbine is rotated by the fluid at a velocity proportional to the volumetric flow rate of the fluid.
Also known to the prior art are mass flow rate flowmeters, one of which includes a measurement assembly comprising a rotatable first member, a second member or impeller for imparting angular momentum to the fluid, and a resilient spring member interconnecting the first and second members. The angular displacement between the first and second members is then used to provide desired information regarding mass flow rate of the fluid. The first member may be driven either by a motor drive assembly, or, by the fluid itself through a turbine connected to the first member. The latter type of mass flowmeter, also known as a motorless mass flowmeter, is seen in U.S. Pat. No. 3,877,304, to Vetsch, issued on Apr. 15, 1975 to the assignee of the present invention.
With respect to volumetric flowmeters and motorless mass flowmeters, the ability of such devices to operate at extremely low flow rates is dependent upon the torque able to be developed by the turbine therein. A significant limitation upon this torque is leakage of the fluid between the peripheral surface of the turbine and the surrounding housing. To reduce the amount of leakage occurring in this manner, it is desirable to reduce the clearance between the turbine's peripheral surface and the housing to a minimum value. This minimum value, however, is limited by dimensional tolerances of the turbine and the housing, inasmuch as the turbine must rotate within the housing without restriction. Further, significant reduction of the clearance presents a more difficult problem in that contaminant particles normally present in any fluid, and particularly present in aircraft fuel supplies, become trapped between the turbine's peripheral surface and the housing, resulting in many cases in stoppage of further rotation of the turbine and consequent malfunction of the flowmeter.
It is therefore an object of this invention to provide an improved fluid flowmeter, of a type including a rotatable member reacting with the fluid whose flow is to be measured, such as a turbine forming part of a measurement assembly, which is capable of operation at very low flow rates.
It is a further object of this invention to provide such an improved fluid flowmeter which, while capable of operation at very low flow rates, has a greatly reduced susceptibility to malfunction due to contaminants present in the fluid.